1. Field of Invention
The present invention relates to an electrically conductive structure of a circuit board, and more particularly to a metal conductive structure disposed in a circuit board for electrically connecting a contact of an electronic device on the circuit board to a power layer and a ground layer of the circuit board. The discrete capacitor may be embedded in the circuit board.
2. Related Art
Usually, an electronic device is disposed on a printed circuit board (PCB) or an IC substrate (hereinafter a circuit board is taken as an example, but the present invention is not limited to the circuit board). The specific contact of the electronic device is electrically connected to a power plane (also referred to as a power layer) and a ground plane (also referred to as a ground layer) in the circuit board through a metal conductive structure in the circuit board, such that the electronic device may receives the power from the circuit board to operate.
With the improvement of the semi-conductor's process technology, the operation speed of the electronic device disposed on the circuit board may be increased quickly, and the switching speed of signals may be also increased. Consequently, the power supply from the circuit board cannot provide a stable power at such a high switching speed, resulting in an unsatisfactory operation of the electronic device. Moreover, a power noise or ground bounce may be induced in the power delivery systems. In order to deal with this problem, persons skilled in the art dispose a decoupling capacitor or a bypass capacitor on the circuit board at the position close to the power/ground pins of the electronic devices. The decoupling/bypass capacitor may be an SMD capacitor or an embedded multilayer planar capacitor. By using of the above skills, the decoupling capacitors provide a localized source of DC power for electronic devices when the signals of electronic devices switch simultaneously. Therefore, the decoupling capacitors may supply energies of power in time when the energies of power is insufficient at a high-frequency operation, thereby absorbing glitch, reducing RF (Radio Frequency) noise and stabilizing the voltage levels.
The similar technique is disclosed in U.S. Pat. No. 5,161,086 entitled “Capacitor laminate for use in capacitive printed circuit boards and methods of manufacture.” Referring to FIG. 1, an electronic device 102 (surface mounted device) is disposed on the surface of the circuit board 100. The electronic device is electrically connected to the power layer and the ground layer in the circuit board through a power wire 104 and a ground wire 106. A planar capacitor 108 is connected between the power wire 104 and the ground wire 106, thereby providing a bypass function of a power system of a surface mount electronic device. Although the structure may achieve the object, an equivalent series inductance (ESL) may be formed between the metal conductive structures (i.e., the power wire 104 and the ground wire 106) when operating at a high frequency. Thus, under this design, the operation frequency is higher than the self-resonant frequency of the capacitor, the parasitic inductance of the planar capacitor and the metal conductive paths will increase, resulting in the impedance also increasing rapidly (which will be illustrated hereinafter).
A method directed to reducing the ESL effect is disclosed in the U.S. Pat. No. 6,678,145 entitled “Wiring connection structure of laminated capacitor and decoupling capacitor, and wiring board”. Referring to FIG. 2, a planar capacitor 120 having a low ESL value is used as a decoupling capacitor on the power supply circuit of a chip 122. In this structure, a plurality of ground via-holes 124 and a plurality of power via-holes 126 (also referred to as feedthrough conductors) are respectively connected to a first-layer inner pole and a second-layer inner pole of the embedded capacitor 120. The feedthrough conductors connected to the first-layer pole and the second-layer pole are adjacently placed so as to depress the magnetic flux produced by the current loops with the opposite directions. Although the patent may reduce the ESL, the ESL 128 of the feedthrough conductors marked at the metal conductive structure in the drawing cannot be reduced effectively.
In the power delivery system of the electronic circuit, the capacitor having the decoupling function includes not only the planar capacitor embedded in the circuit board, but also a surface mount device (SMD) capacitor. However, since the embedded planar capacitor is closer to the power and ground pins of the electronic device as compared with the SMD capacitor welded on the surface of the PCB or the IC substrate, the parasitic inductance produced on the power transmission path of the embedded capacitor at a high speed of signal switching is lower than that produced by the SMD capacitor. That is, the decoupling effect of the embedded planar capacitor is better. But, similar to the common capacitor, along with the increasing signal switching speed of the electronic device of the planar capacitor, the parasitic inductance increases, resulting in degenerating the decoupling effect of the decoupling capacitor that is originally provided on the low-impedance path to depress the high-frequency undesired noises.